A skyline is the outline or shape viewed near the horizon. It can be created by a city’s overall structure, or by human intervention in a rural setting, or in nature that is formed where the sky meets buildings or the land.
City skylines serve as a pseudo- fingerprint as no two skylines are alike. For this reason, news and sports programs, television shows, and movies often display the skyline of a city to set a location. The term The Sky Line of New York City was first introduced in 1896, when it was the title of a color lithograph by Charles Graham for the color supplement of the New York Journal.  Paul D. Spreiregen, FAIA, has called a [city] skyline "a physical representation [of a city's] facts of life ... a potential work of art ... its collective vista." 
Towers from different eras make for contrasting skylines.
Some remote locations have striking skylines, created either by nature or by sparse human settlement in an environment not conducive to housing significant populations.
Several services rank skylines based on their own subjective criteria. Emporis is one such service, which uses height and other data to give point values to buildings and add them together for skylines. The three cities it ranks highest are Hong Kong, Seoul and Shenzhen. 
When Charles Graham's view of New York was published, the new term used in the title, "sky line," caught on immediately.
geographers have tended to neglect the substantial impact of skyscrapers on urban life.